1. Field of the Invention
The invention is in the field of information signal privacy systems and particularly signal coders and decoders which scramble and descramble, respectively, the information signal.
2. Description of the Prior Art
An information signal scrambler system operates to convert the analog information, which may be speech and for ease in explanation will hereinafter be referred to as speech, into a non-intelligent garble prior to transmission, thereby preventing unauthorized deciphering of the speech communication over an exposed transmission link. At the receiver, the speech is unscrambled to recover the information content. Such systems are particularly applicable to military and civil law enforcement communication channels where channel security is a major concern. For example, in battlefront conditions, orders from the command station to front line troops must be kept secret since the enemy normally attempts to eavesdrop on these communications.
Various speech scrambler systems are currently available to offer communication privacy. Some of these use spectrum folding, others use inversion techniques, while still others use combinations of these techniques. The spectrum manipulation usually employs single-sideband techniques with the inherent requirement for at least two narrowband, single-sideband filters--often mechanical. The scramblers are, therefore, costly and not amenable to micro-miniaturization by using LSI technology. More specifically, certain of the presently available scrambler systems utilize a technique which provides for the frequency and/or phase shifting of portions of the analog speech signal. At the receiver, the scrambled signal is unscrambled by shifting its frequency and/or phase back to its original position. However, due to the inherent limitations of such systems with respect to the total number of frequencies and phase shifts available, these systems are relatively unsecure. That is, it is relatively easy for an unauthorized listener to unscramble the transmission.
In attempts to improve upon scrambler systems, attempts have been made to digitize the speech and modulate it with a digitized pseudorandom code. However, such systems have been found to be complicated and expensive and further require a bandwidth much greater than is normally available for either radio or telephone transmission of the unscrambled speech.